The separation of solutes from organic solvents is desirable in many processes. It would be useful to have a reverse osmosis membrane that was insoluble in all organic solvents, and showed a high rejection for various solutes. Such a membrane could be useful in purifying streams that would degrade or dissolve many other membranes.
Interfacially polymerized membranes were initially discovered in the 1970's for use in water desalination (see "In Situ-formed Condensation Polymers for Reverse Osmosis Membranes: Second Phase", North Star Research Institute, prepared for Department of the Interior, July 1974, available from NTIS, report #PB-234 198; "Continued Evaluation of In Situ-formed Condensation Polymers for Reverse Osmosis Membranes", Midwest Research Institute, prepared for Office of Water Research and Technology, April 1976, available from NTIS, report #PB-253 193; "Interfacially Synthesized Reverse Osmosis Membrane", U.S. Pat. No. 4,277,344, Jul. 7, 1981, assn. to Film Tec Corporation). Prior art only describes the use of these membranes for the separation of aqueous solutions by reverse osmosis. There is no mention of the use of these membranes for the separation of solutes from organic solvents by reverse osmosis.
Interfacially polymerized membranes are composed of a highly crosslinked and generally insoluble condensation polymer which is formed in situ on a micro-porous film. Most of these membranes are formed with di- or polyamines which are reacted with multi-functional iso-cyanates or acid chlorides. Amines react very readily with both of these reactants. Several of these membranes have been commercialized for water desalination purposes by companies such as UOP, Film Tec and Desalination Systems Inc. All of the commercial membranes use a polysulfone ultrafiltration membrane (0.02 to 0.1 micron pore size) for the microporous support film. Prior art does describe the use of some other microporous support films such as polyvinylchloride ultrafiltration membranes but none of the support films mentioned are particularly resistant to organic solvents.
These membranes are formed using the following procedures. A thin layer of a dilute solution of one component, usually an aqueous solution of the amine, is put on one side of the microporous support film. A thin layer of a dilute solution of the second component, usually in a water immiscible solvent, is then put on top of the water solution layer. The order of applying the solutions can be reversed. The two components react at the water/solvent interface forming a thin (less than 1 micron thick) highly crosslinked polymer layer. This polymer layer is the active layer of the membrane at which separation occurs. Some examples of formulations mentioned in the prior art are reacting polyethylenimine with toluene diisocyanate, reacting polyethylenimine with isophthaloyl dichloride and reacting m-phenylene diamine with trimesoyl chloride.
These membranes exhibit high salt rejections from water (&gt;95%). The commercially available membranes prepared on polysulfone ultrafiltration membranes are not suitable for separating solutes from organic solvents as these typically soften or dissolve polysulfone.
French patent 2,595,370 teaches a multiple effect extraction process using counter current solvent flow. The process utilizes a main column separated into 2 zones by a draw off tray and a second column which fractionates the side stream drawn off from the first column. The fractionation zone produces an over head raffinate which is fed back to the top zone of column 1 above the draw-off tray. The bottoms from the fractionation zone are cooled and separate into a pseudo raffinate and an extract. This extract is recycled to the bottom zone of column 1 just below the draw-off tray. It can optionally also be fed into the top zone of column 1 just above the draw-off tray. By this scheme a raffinate is recovered from the top of the first column, an extract from the bottom of said column and a pseudo raffinate from the separation zone to which the bottoms fraction from the fractionation zone is fed.
In an alternate embodiment the extract from the bottom of column 1 can be cooled to salt-out in a separation zone an upper phase of lighter hydrocarbons which is recycled back to the bottom of the bottom zone of column 1. The bottoms fraction from this separation zone is a true extract phase.
French Patent 2,595,371 teaches a process for the selective solvent extraction of a hydrocarbon mixture. Solvent is passed counter currently to the hydrocarbon feed employing 2 or more separation columns resulting in the separation of the feed into a raffinate, a pseudo-raffinate and an extract. Feed is introduced into a first column while fresh solvent is introduced into the top of a second column. The overheads from the first column constitute the feed to the second column. The bottoms from the second column are cooled and permitted to salt-out in a separation zone wherein an upper phase pseudo raffinate is recovered and a bottom phase of recycle solvent is recovered. This bottom phase recycle solvent is used as the solvent introduced into the first column. Extract is recovered from the bottom of the first column and raffinate from the top of the second column. In an alternative embodiment part of the pseudo raffinate can be cycled back to the bottom of the second column while the extract from the first column can be cooled to salt-out in a separation zone producing a upper phase of lighter hydrocarbon which is recycled to the bottom of the first column, and a true extract bottoms phase.
U.S. Pat. No. 4,311,583 teaches a solvent extraction process. A hydrocarbon feed is contacted with N-methyl pyrollidone in an extraction zone. The primary extract is separated into a secondary raffinate and a secondary extract by cooling the primary extract optionally with the addition of water or wet solvent. The secondary raffinate is separated from the secondary extract. At least part of the secondary raffinate is combined with the primary raffinate to obtain an increased yield of desired quality raffinate oil product. A part of the secondary raffinate may be returned to the lower part of the extraction zone.
U.S. Pat. No. 4,328,092 teaches the solvent extraction of hydrocarbon oils. The process uses N-methyl-2-pyrollidone. The extract from the solvent extraction zone is cooled to form two immiscible liquid phases, a secondary extract phase and a secondary raffinate phase. The secondary raffinate phase is recycled to the extraction zone resulting in increased yield of refined oil product and in energy savings.
"Liquid Extraction" 2d Ed, R. E. Treybol, McGraw-Hill Book Company, 1963 pgs 144-145, 270-273. This reference shows that extractor reflux has been practiced and that reflux for extraction operations is obtained by distillation methods, chilling or by the addition of an anti solvent.